Heat-sensitive recording media have a heat-sensitive color-developing layer formed on a support base, for example, by applying an aqueous dispersion containing a leuco dye, a color developing agent and a dispersant on the support base and drying the applied aqueous dispersion. When such a heat-sensitive recording medium is heated, the leuco dye and the color-developing agent are melted and mixed together to react with each other, whereby the heat-sensitive color-developing layer develops a color. There has been developed a heat-sensitive recording medium which employs a polyvinyl alcohol (hereinafter abbreviated as PVA) containing a sulfonic acid group (modified with sulfonic acid) as the dispersant for dispersing the leuco dye and the color-developing agent (see PLT1).
The heat-sensitive recording media are relatively inexpensive, and permit easy maintenance of a recording device. Therefore, the heat-sensitive recording media are used as various recording media such as facsimile paper sheets, POS labels and various tickets. In recent years, the heat-sensitive recording media are often used outdoor and, therefore, exposed to sunlight more often.
When the heat-sensitive recording media are exposed to ultraviolet radiation, however, the chemical structures of the color developing agent, the leuco dye, a sensitizer and the like are changed to develop the color. Problematically, this results in so-called background fogging.
In order to solve the problem associated with light, a technique for providing an intermediate layer containing an aqueous UV-absorbing polymer between the heat-sensitive recording layer (heat-sensitive color-developing layer) and a protection layer has been proposed (see PLT2).
However, a heat-sensitive recording medium according to this technique is insufficient in light resistance. This is supposedly because the leuco dye and the like to be influenced by the light and the UV absorbing agent are contained in different layers, which are spaced from each other.
It is undesirable that the heat-sensitive recording layer (heat-sensitive color-developing layer) contains an organic compound such as the UV absorbing agent. This is because the leuco dye and the color developing agent are liable to be dissolved in the organic compound to react with each other to develop the color, resulting in the background fogging.